1. Field of the Invention
The present invention relates to an abscissa calibration jig and an abscissa calibration method of a laser interference measuring apparatus. Particularly, it relates to calibration of a position on a measurement plane in a laser interference measuring apparatus having a convergent optics.
2. Description of the Related Art
Apparatuses called laser interferometers or laser interference measuring apparatuses have been heretofore used widely for precisely measuring the surface texture or the surface shape of an object to be measured. A measured value obtained by the laser interference measuring apparatus is acquired as a measured value of the height of the surface of the object to be measured. The measured value is used as feedback data for evaluation of the shape of the surface of the object to be measured, the step of processing the object to be measured, or the like. In such an application, it is necessary to accurately know the position to which measured data obtained by the laser interference measuring apparatus corresponds on the surface of the object to be measured, that is, the abscissa (two-dimensional position perpendicular to the optical axis of laser light). That is, when there is an abnormal value in the measured data, correction or the like cannot be performed accurately if the point where the abnormal value has occurred cannot be identified precisely.
In a laser interference measuring apparatus according to the background art, an image of interference fringes is generally acquired by a two-dimensional light receiving unit such as a CCD camera. Therefore, the aforementioned abscissa is evaluated in the laser interference measuring apparatus based on the position to which each light receiving element arrayed two-dimensionally in the light receiving unit such as a CCD camera corresponds on the surface of the object to be measured. The correspondence relation in the abscissa between the surface position of the object to be measured and the light receiving unit may get out of an ideal correspondence relation due to image distortion caused by the properties of an optics contained in the laser interference measuring apparatus, or the influence of a difference between longitudinal and lateral magnifications.
In an application in which the laser interference measuring apparatus measures a planar shape, the abscissa can be evaluated or correction or the like using an evaluated value of the abscissa can be performed when a standard or the like having a known shape like a two-dimensional array is measured.
On the other hand, when the spherical surface shape of an object to be measured is measured using a convergent optics, image distortion is also produced when the spherical shape is developed into a planar image in a two-dimensional imaging device, in addition to the image distortion or the difference between longitudinal and lateral magnifications. For example, there is a little change when an image located near the equator of the spherical surface is projected onto a plane, but there is a greater difference when an image located more distantly from the equator is projected onto a plane. A spherical surface shape is developed thus on a plane by nonlinear conversion. Therefore, more careful evaluation of the abscissas is required.
A method disclosed in JP-A-2007-327892 or JP-A-2002-206915 has been proposed as a method for correcting the abscissas in an interference measuring apparatus when the case where such a convergent optics is used is taken into consideration. JP-A-2007-327892 has given description to a method in which an object to be inspected and a light shielding plate having a contour shape calibrated in advance are measured integrally and the measured shape of the light shielding plate and the calibrated shape are compared to evaluate the abscissas. JP-A-2007-327892 has given description to an evaluation method corresponding to an object to be inspected having a spherical surface.
JP-A-2002-206915 has given description that a reflective optical element having a predetermined pattern is placed on a plane in a position where an object to be inspected is placed, and a measured value of the pattern is compared with the calibrated value so as to calibrate the abscissa. In JP-A-2002-206915, a reflection portion of the reflective optical element is a diffraction grating so that the angle of reflection of light thereof is made the same as that in the case where an object to be inspected having a spherical surface is measured. In this method, an abscissa error having nonlinearity can be evaluated.
However, the contour shape is limited to a circle in the case of JP-A-2007-327892. Such a method has an effect in evaluation of a lower-order abscissa error such as a difference between longitudinal and lateral in magnifications, but has a limited effect in evaluation of an abscissa error which has a higher-order term or which is nonlinear. On the other hand, according to JP-A-2002-206915, an abscissa error having nonlinearity can be evaluated. Even in the present level of technology, however, it is supposed that production of a diffraction grating as in JP-A-2002-206915 involves great difficulty.
For example, assume that ρ designates a distance between a point on a reflective optical element and the center of the element, and θ designates an angle between a light beam of convergent luminous flux reaching the point and the optical axis of an interferometer. In this case, the angle of reflection required in a concentric position where the distance from the center of the element is ρ is expressed by θ. In addition, the relation in which ρ is proportional to TAN θ is established between the distance ρ and the angle θ. That is, the reflective optical element is required to have an optical characteristic in which the angle of reflection changes smoothly in proportion to TAN θ in accordance with the distance ρ from the center. It is not easy to manufacture such an element strictly. Further, when such a planar object to be measured is placed in a convergent optics, it is difficult to adjust the focus of the interferometer all over the visual field of an image. There may be considered a problem that it is difficult to perform precise measurement for evaluating the abscissa.